1. Field of the Invention
The present invention relates to a radio receiver and particularly to a gain controlled amplifier for use in a radio receiver. The radio receiver or gain controlled amplifier may be fabricated from discrete components or as an integrated circuit. In its integrated form, the radio receiver and the gain controlled amplifier can be used in battery powered communications apparatus such as digital pagers and cellular/cordless telephones.
2. Description of the Related Art
For convenience of description the present invention will be described with reference to an integrated receiver for use in a digital pager.
Frequently low power receivers used in applications such as digital paging do not incorporate automatic gain control (AGC). However there are situations, such as when receiving signals in an environment where there are a plurality of very high powered transmitters on nearby channels within the receiver RF bandwidth, that the receiver circuit can be saturated. When the wanted signal amplitude is high enough to allow RF gain reduction AGC provides a means of reducing the received input signal to manageable levels based on the fact that unwanted products caused by large interferers reduce by square or cubic laws, whereas the wanted signal decreases linearly. It is known from European Patent Specification EP-A-0 795 967 to provide AGC to the input stage of a radio receiver to attenuate excessively large input RF signals. This known solution requires a PIN diode which is not readily integratable.